Add unit test framework for Vulkan

[doug-walker]

Now that OCIO 2.5 supports Vulkan, it would be great to test that in GitHub Actions. This would probably consist of several tasks, which could potentially be done by different people:

• Add to libutils a "vulkanapp" equivalent to the existing oglapp or metalapp. The sample code below may have the necessary pieces.
• Update tests/gpu/GPUUnitTest.cpp to add an option to execute the existing tests through that renderer (should be trivial).
• Update the GitHub GPU Action to do whatever magic is needed to make Vulkan run properly in the virtualized instance.

JF provided the following example, which may help. "This code sample apparently demonstrates headless rendering in Vulkan, which is supported natively:" https://github.com/SaschaWillems/Vulkan/blob/master/examples/renderheadless/renderheadless.cpp